Modulation interferometer and fiberoptically divided measuring probe with light guided

ABSTRACT

An interferometric measuring device for detecting the shape, roughness or distance of surfaces is described. The interferometric measuring device has a modulation interferometer in which two partial beams are formed, one of which is shifted in its light phase or light frequency with respect to the other by a modulation device. The surface is measured with a measuring probe which is connected to the modulation interferometer and in which a measuring beam and a reference beam are formed, and an interference pattern which is analyzed in a connected receiving unit is formed from the measuring beam and the reference beam. A compact design that is easy to handle even in a manufacturing process is achieved by spatially separating the modulation interferometer which is designed as a basic unit from the measuring probe and by the fact that it can be connected to the measuring probe by an optical fiber arrangement, and the measuring arm and the reference arm are formed by solids conducting the measuring beam and the reference beam.

FIELD OF THE INVENTION

The present invention relates to an interferometric measuring device fordetecting the shape, roughness or distance of surfaces by using amodulation interferometer having a spatially coherent beam source and afirst beam splitter for splitting its beam into two partial beams, oneof which is shifted in its light phase or light frequency with respectto the other by a modulation device and then the two partial beams arecombined, having a measuring probe in which the combined partial beamsare split into a measuring beam guided through a measuring arm andreflected on the surface and a reference beam guided through andreflected in a reference arm, and in which the reflected reference beamis superimposed on the reflected measuring beam, and having a receivingunit for splitting the superimposed beam into at least two beams havingdifferent wavelengths and converting the beams into electrical signals,and for analyzing the signals on the basis of a phase difference.

BACKGROUND INFORMATION

An interferometric measuring device is known from European Patent No.126 475. In this known measuring device, rough surfaces of a measuredobject are measured interferometrically, a beam gun unit having laserlight sources which emit light of different wavelengths being used. Thelaser light is divided into a reference beam of a reference beam pathand a measuring beam of a measuring beam path using a beam splitter. Themeasuring beam path impinges on the surface to be measured, while thereference beam path is reflected on a reference surface, for example inthe form of a mirror. The light reflected from the surface and thereference surface is combined in the beam splitter and focused, with thehelp of a lens, in an interferogram plane, where a speckle pattern isobtained. This speckle pattern is analyzed to determine the surfaceshape, a phase difference of the interferogram phases in the measuringpoint being determined. In order to simplify the analysis, a heterodynemethod is used, the frequency of the reference beam being shifted withrespect to the frequency of the measuring beam by a heterodyne frequencyusing a frequency shifter in the reference beam path. With thismeasuring device, a fine resolution of the surface shapes can beobtained.

However, the adjustment and handling are complicated for use inindustrial manufacturing, for example.

Another interferometric measuring device is described in GermanPublished Patent Application No. 39 06 118, in which optical fibers areprovided between a plurality of laser light sources and a measuringsection. Here again, a phase difference is evaluated for determining thesurface structures. This known design is also disadvantageous withregard to the handling and adjustment in places that are difficult toaccess.

Another interferometric measuring device, as described in GermanPublished Patent Application No. 198 08 273, which was not publishedpreviously, this device having a modulation interferometer and aspatially separate measuring probe connected to it by an optical fiberarrangement, is much more favorable for practical use in a manufacturingprocess for example. Advantages include in particular a short-termcoherent radiation source of the modulation interferometer, yielding astable beam that can be analyzed well, and the relatively compact designof the measuring probe. However, the arrangement of the measuring probewith respect to the surface of the measured object is still associatedwith some adjustment measures which would make a simplification seemdesirable.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an interferometricmeasuring device of the yielding simplified handling with a relativelysimple, compact design.

Accordingly, the modulation interferometer designed as a basic unitwhich is spatially separate from the measuring probe and can beconnected to it by an optical fiber arrangement, and the measuring armand the reference arm are formed by solids conducting the measuring beamand the reference beam.

The measuring probe which is spatially separate from the modulationinterferometer, which is designed as a basic unit, by the optical fiberarrangement is itself compact due to the design of the measuring arm andthe reference arm as a solid and thus it is easy to handle and isdesigned to be easily adjustable with respect to the measured object. Itis easy to adjust the reference arm and the measuring arm of themeasuring probe relative to one another and with respect to theinterferometric measuring device because of the unambiguous positioningof the individual elements.

If the design is such that a collimator device is provided at the inputof the measuring probe and a focusing device is provided at the outputof the measuring arm, and a deflecting element downstream from thefocusing device is provided for output and then re-injection of themeasuring beam directed at and reflected by the surface to be measured,then a favorable measuring beam and reference beam are obtained to formthe interference pattern, and on the other hand the measuring beam canbe aligned perpendicular to the measuring arm due to the deflectingelement even when the surface to be measured is at an inclination to thedirection of the measuring arm, so that a reliable measurement of thesurface is achieved. An advantageous embodiment involves the collimatordevice and/or the focusing device being a GRIN lens.

Interference of the two partial beams before entering the measuringprobe is prevented by the fact that one of the two partial beams in themodulation interferometer passes through a delay element which generatesa difference in the optical path lengths of the two partial beams whichis greater than the coherence length of the beam emitted by the shortcoherent beam source, and another difference in the optical path lengthsis generated in the measuring arm with respect to the reference arm,compensating for the difference in optical path lengths generated by thedelay element, and this interference comes about only after reflectionat the surface or in the reference arm and thus coherence multiplexingis made possible.

Multiple sections of the surface to be measured or multiple separatesurfaces can be measured rapidly and reliably without repositioning themeasuring probe by the fact that the measuring arm has at least oneadditional deflecting element with which the measuring beam guided inthe measuring arm is split and directed at another site on the surfaceto be measured, and the measuring beam reflected by this surface isinjected back into the measuring arm. There are two different designpossibilities here due to the fact that different optical path lengthscan be preselected in the modulation interferometer by differentinterchangeable delay elements, and the compensating difference in theoptical path lengths is formed by adjusting a reflecting or deflectingelement of the reference arm or due to the separate reference armsassigned to the individual measuring beams split off in coordinationwith the different delay elements. If separate reference arms areprovided, then the selected measuring site on the surface is obtainedunambiguously by allocation to the corresponding delay element withoutfurther adjustment through coherence multiplexing. However, it isrelatively simple to adjust the compensating optical path difference ofthe measuring arm with respect to the reference arm by the reflectingand deflecting element of the reference arm.

Two different design options for the measuring probe consist of the factthat the measuring arm and the reference arm(s) are designed as separatearms of the measuring probe in the manner of a Michelson interferometer(as illustrated in FIGS. 1 and 2) or in a common arm in the manner of aFizeau interferometer (as illustrated in FIG. 3). If the measuring armand the reference arm here are designed in a common arm of the measuringprobe, this yields an especially compact design of the measuring probe,which can be used even under unfavorable space conditions.

A further simplification of the design of the measuring device isachieved due to the fact that the beam directed toward and away from themeasuring probe is passed over a common monomode optical fiberarrangement, and the beam sent to the receiving unit is output from theoptical fiber arrangement by an arm section. Coupling of the measuringprobe to the modulation interferometer and the receiving unit can beaccomplished easily by using plug connectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an interferometric measuring devicehaving a measuring probe which is spatially separate from a modulationinterferometer.

FIG. 2 shows another embodiment of the interferometric measuring devicehaving a plurality of measuring beams and reference beams.

FIG. 3 shows another embodiment of the interferometric measuring device,where the measuring arm and the reference arm are designed differently.

DETAILED DESCRIPTION

The embodiments of an interferometric measuring device illustrated inFIGS. 1 through 3 for determining the shape, roughness or distance ofsurfaces each have a modulation interferometer 1 and a measuring probe 2spatially separate from the interferometer and coupled to it by anoptical fiber arrangement 4 and also a receiving unit 3 to receive thebeam returned by the measuring probe.

Modulation interferometer 1 has a beam source 11, preferably in the formof a short coherent broad-band beam source 11 having a continuousradiation distribution of a plurality of different wavelengths with goodspatial coherence at the same time, such as a superluminescence diode.The beam from beam source 11 is collimated by a collimator lens 12 andsplit by a first beam splitter 13 into two partial beams, each of whichis passed through acousto-optical modulators 14, 14′ and combined againby deflector mirrors 15, 15′ at a second beam splitter 13′, with one ofthe two partial beams being passed through a delay element 16 or 16′(see FIGS. 2 and 3). The combined partial beams pass through a lenssystem 17 into monomode optical fiber arrangement 4.

The two partial beams are modulated by acousto-optical modulators 14 or14′ with adjacent but different frequencies. An optical path differencein the two partial beams is created in the modulation interferometer,e.g., of the Mach-Zehnder type, by delay element 16 or 16′, which delayelement 16, 16′ is designed as a glass plate of a certain thickness, forexample. In this way, the partial beams which are combined by beamsplitters 13′ which are designed as semitransparent panes, for example,are present as separate wave trains having two adjacent but differentfrequencies and are shifted spatially toward one another with a lengthgreater than the coherence length of beam source 11.

Measuring probe 2 according to FIGS. 1 and 2 is designed as aninterferometer of the Michelson type. The combined light beam sent overmonomode optical fiber arrangement 4 is collimated by a collimatordevice 21 in the form of a lens system, then split by a third beamsplitter 22 into a measuring beam and a reference beam. The measuringbeam is focused at the output end of a measuring arm 211 carrying themeasuring beam by a focusing device 23 in the form of a lens system anddeflected at the output of measuring arm 211 by a deflecting element 24in the form of a prism in such a manner that at the outlet of the prism,the axis of the beam cone is directed perpendicularly onto the surfaceto be measured. The unit of prism 24 and lens system 23 can be replaced,so that surfaces having a different contour can be measured. Collimatordevice 21 at the input of measuring probe 2 and focusing device 23 inthe end area of measuring arm 211 are preferably designed as GRIN(=grade index) lenses which offer favorable beam guidance.

The reference beam separated at the third beam splitter is sent in areference arm 212 to a reflector element 28 in the form of a prismprovided at the end of the reference arm, where it is deflected, theprism being adjustable in the direction of the optical axis of referencearm 212. Measuring arm 211, carrying the measuring beam as collimatedbeam 25, then has a measuring beam conducting body 26 in the form of asolid at third beam splitter 22, while reference arm 212 has a referencebeam conducting body 27 or 27′ in the form of another solid.

The length of collimated beam 25 in measuring arm 211 and its diameterare adapted to the dimensions of the measured object, e.g., the depthand diameter of a borehole to be measured. The section in which beam 25is collimated may be designed as a glass cylinder, so that measuringprobe 2 has a compact design.

The reference beam passes through a medium 27 of a great dispersion,such as a piece of glass, to compensate for the chromatic dispersion ofmeasuring arm 211 and delay element 16 of modulation interferometer 1.The reference beam is returned by reflector 28, such as a mirror or acatadioptric element, in which case a compensating optical pathdifference between measuring arm 211 and reference arm 212 can beadjusted by adjustment of reflector 28 to compensate for the opticalpath difference created by delay element 16.

The measuring beam returned over measuring arm 211 and the referencebeam returned over reference arm 212 interfere at third beam splitter 22and are returned over optical fiber arrangement 4, which also serves todirect the combined beam to measuring probe 2, and are sent over an armsection 41 to receiving unit 3. Optical fiber arrangement 4 can beconnected by plug connector 42, 42′ to measuring probe 2 on the one handand to modulation interferometer 1 and receiving unit 3 on the otherhand. The plug connector of modulation interferometer 1, measuring probe2 and/or receiving unit 3 may be arranged as a corresponding jackdirectly on the housing.

Receiving lens system 31 of receiving unit 3 causes the emission area ofmonomode optical fiber arrangement 4 to be imaged in the plane of aphotoreceiving unit 33 after passing through a beam splitter 32. Eachphotodiode thus receives the image of the emission area of the opticalfiber arrangement with a given wavelength.

As FIG. 2 shows, another beam splitter 29 in the form of a prism withwhich another measuring beam is split off is arranged in measuring arm211. In comparison with the first continuous measuring beam, thisadditional measuring beam has a different output angle, so that surfaceareas of different orientations can be measured at the same time, withthe position of the measuring probe with respect to the measured objectbeing retained and no additional adjustment work being required. Withrespect to the additional measuring beam, this yields another opticalpath difference between measuring arm 211 and reference arm 212 whichcan be compensated by replacing delay element 16 with another delayelement 16′ of a suitably adjusted optical path difference. An accurateadjustment of the compensating optical path difference in measuringprobe 2 can be performed, for example, by adjustment of reflector 28, sothat the measuring beam split off is brought to interference with thereference beam. The measurement site can be identified by delay element16 or 16′ used and by coherence multiplexing. According to FIG. 2,however, a second reference arm having a second reference beamconducting body 27′ and a second reflector 28′ is formed to produce acompensating optical path difference coordinated with delay element 16′which has been replaced, so that two fixedly predetermined compensatingoptical path differences are obtained according to the two measuringbeams and measuring sites coordinated with the optical path differencesof delay elements 16, 16′, although the compensating path differencesare still to adjusted by a precision adjustment of reflector 28 or 28′.Two third beam splitters 22 and 22′ are provided to form the tworeference arms.

Accordingly, multiple measuring beams may also be formed by additionalbeam splitters 29, in which case a part of the measuring beam is alwayspassed through beam splitter 29 to the beam splitter arranged behind itor the outlet of measuring arm 211 without deflection. The number ofreference arms then preferably corresponds to the number of measuringbeams formed, and a corresponding number of delay elements are alsoprovided in modulation interferometer 1, so that there is a definitecorrelation with the measuring site by coherence multiplexing.

Another embodiment of the interferometric measuring device is shown inFIG. 3. The operation here corresponds to that according to FIG. 2. Incontrast with the embodiment according to FIG. 2, measuring probe 2 isdesigned as an interferometer of the Fizeau type where a GRIN lens 21 isagain provided in the form of a glass cylinder at the input of measuringprobe 2 downstream from an optical fiber 26, for example, and GRINlenses 21, 23, 21′ and 23′ are also arranged accordingly in the end areaof the measuring arm. Upstream from the uncoupling point of the firstmeasuring beam, a semitransparent optical element on which a portion ofthe beam guided into measuring probe 2 is reflected is arranged in thearea of the input GRIN lens. Interference with the measuring beam splitoff at the first point in the path of the beam and reflected back fromthe surface to be measured takes place in this semitransparent element,in which case the optical path difference between the measuring beam andthe reference beam formed on the semitransparent optical element is sogreat that the path difference of delay element 16 provided inmodulation interferometer 1 is compensated. Accordingly, in the GRINlens arranged in the beam path downstream from the splitting point ofthe first measuring beam is also arranged a semitransparent opticalelement 28.1′ on which an interference is created in the mannerdescribed above with the measuring beam guided over the end area ofmeasuring probe 2. The compensating optical path difference formedbetween this additional semitransparent optical element 28.1′ and therespective measuring beam is matched to another delay element 16′arranged in modulation interferometer 1. Therefore, the measurementsites can also be identified in this design of the interferometricmeasuring device on the basis of coherence multiplexing based on opticaldelay element 16 or 16′ used here.

Semitransparent optical elements 28.1, 28.1′ are designed to be planarand perpendicular to the optical axis of the measuring beam and they maybe arranged in a focusing point 28 of the optical beam or in a plane inwhich the beam passing through measuring arm 211 is collimated. A singlereference beam may be provided for all measuring beams (or outputprisms, i.e. a corresponding number of reflecting faces, of themeasuring beams), i.e., a single reflecting face may be provided, or thesame number of reference beams as measuring beams or output prisms maybe provided. The optical path of the reference beam can be adjusted byvarying the thickness of delay element 16, 16′ in modulationinterferometer 1. The thickness can be varied, for example, by rotatingthe pane of glass of delay element 16, 16′ or by exchanging two platesof glass. The chromatic dispersion can be compensated by using a panel18, 18′ of a highly dispersive material, such as a plate of glass and agiven thickness, in the other arm of modulation interferometer 1. Tomeasure a given surface, it is sufficient to use a prism adapted to theprofile of the surface to be measured and to use a corresponding glassplate as delay element 18, 18′ or for compensation of chromaticdispersion in modulation interferometer 1.

The optical beam power of measuring beam in measuring arm 211 is usuallymuch lower than the optical beam power of the reference beam inreference arm 212. It is therefore advantageous to design third beamsplitter 22 or 22′ to be asymmetrical to obtain an increased beam powerof the measuring beam reflected back comparable to that of the referencebeam.

In the case of the embodiment according to FIG. 3, it is advantageousthat, except for the slender form, measuring probe 2 is less sensitiveto changes in temperature due to the measuring arm and the reference armrunning in a common arm.

One advantage of all embodiments is that the adjusting and regulatingdevice of measuring probe 2 in modulation interferometer 1 is separatefrom the measuring probe. The same adjusting and regulating device canbe used for a large number of measuring probes 2, thus making themeasuring device cost effective.

What is claimed is:
 1. An interferometric measuring device for detectingone of a shape, a roughness, and a distance of a surface, comprising: amodulation interferometer designed as a basic unit and including: aspatially coherent beam source, a first beam splitter for splitting abeam into a first partial beam and a second partial beam, and amodulation device, wherein: one of a light phase and a light frequencyof one of the first partial beam and the second partial beam is shiftedwith respect to another one of the first partial beam and the secondpartial beam by the modulation device, and the first partial beam andthe second partial beam are combined; an optical fiber arrangement; ameasuring probe that is spatially separate from the modulationinterferometer and connectable to the modulation interferometer via theoptical fiber arrangement, the measuring probe including: a measuringarm, and a reference arm, wherein: the combined first partial beam andthe second partial beam are split into a measuring beam guided throughthe measuring arm and reflected on the surface and a reference beamguided through and reflected in the reference arm, the measuring arm isformed by a solid conducting the measuring beam, the reference arm isformed by a solid conducting the reference beam, and the reflectedreference beam is superimposed on the reflected measuring beam toproduce a superimposed beam; and a receiving unit for: splitting thesuperimposed beam into at least a third beam and a fourth beam with awavelength that is different than a wavelength of the third beam,converting the third beam and the fourth beam into correspondingelectrical signals, and analyzing the corresponding electrical signalson the basis of a phase difference.
 2. The measuring device according toclaim 1, further comprising: a collimator device arranged at an input ofthe measuring probe; a focusing device arranged at an output of themeasuring arm; and a deflecting element arranged downstream from thefocusing device and for outputting and re-injecting the measuring beamdirected at and reflected by the surface to be measured.
 3. Themeasuring device according to claim 2, wherein: at least one of thecollimator device and the focusing device is a GRIN lens.
 4. Themeasuring device according to claim 1, further comprising: a delayelement through which one of the first partial beam and the secondpartial beam passes and for generating a difference with respect to afirst optical path length of the first partial beam and a second opticalpath length of the second partial beam, the difference being greaterthan a coherence length of the beam emitted by the short coherent beamsource, wherein: another difference in the first optical path length andthe second optical path length is generated in the measuring arm withrespect to the reference arm in order to compensate for the differencegenerated by the delay element.
 5. The measuring device according toclaim 1, wherein: the measuring arm includes at least one additionaldeflecting element with which the measuring beam guided in the measuringarm is split and directed at another site on the surface to be measured,and the measuring beam reflected by the other site on the surface isinjected back into the measuring arm.
 6. The measuring device accordingto claim 5, further comprising: a structure arranged with respect to thereference arm and including one of a reflecting element and a deflectingelement; and a plurality of different interchangeable delay elements forselecting in the modulation interferometer different optical pathlengths, wherein: a compensating difference in the different opticalpath lengths is formed by adjusting the structure including the one ofthe reflecting element and the deflecting element of the reference arm.7. The measuring device according to claim 5, further comprising: aplurality of different interchangeable delay elements for selecting inthe modulation interferometer different optical path lengths, wherein:the reference arm corresponds to separate reference arms, the measuringbeam corresponds to individual measuring beams, and a compensatingdifference in the different optical path lengths is formed by theseparate reference arms assigned to the individual measuring beams splitoff in coordination with the plurality of different interchangeabledelay elements.
 8. The measuring device according to claim 1, wherein:the measuring arm and the reference arm are designed as separate arms ofthe measuring probe in accordance with a Michelson interferometer. 9.The measuring device according to claim 1, wherein: the measuring armand the reference arm are designed as a common arm in accordance with aFizeau interferometer.
 10. The measuring device according to claim 1,further comprising: a common monomode optical fiber arrangement; and anarm section, wherein: a beam directed toward and away from the measuringprobe is passed over the common monomode optical fiber arrangement, anda beam sent to the receiving unit is output from the common monomodeoptical fiber arrangement by the arm section.